Variable focal position optical system and light beam scanning apparatus

ABSTRACT

A variable focal position optical system which includes a variable focal length lens subsystem whose focal length is variable and a fixed focal length lens subsystem whose focal length is fixed. The variable focal length lens subsystem and the fixed focal length lens subsystem are arranged such that a distance between a principal point of the variable focal length lens subsystem and a principal point of the fixed focal length lens subsystem at a variable focal length lens subsystem side is substantially equal to the focal length of the fixed focal length lens subsystem. As a result, it is possible to change only a focal position with substantially no change to a beam diameter at the focal position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable focal position opticalsystem and to a light beam scanning apparatus, and in particular, to avariable focal position optical system in which the focal position of anincident light beam is varied, and to a light beam scanning apparatuswhich includes the variable focal position optical system.

2. Description of the Related Art

Laser computer output microfilmers (laser COMs) are known as devices forrecording information such as characters or images onto a recordingmaterial by a light beam. In the laser COM, for example, a laser beam isscanned on the basis of information outputted from a computer, andinformation such as characters is directly recorded onto a recordingmaterial such as a microfilm (Japanese Patent Application Laid-Open No.55-67722). In this type of laser beam recording device, after the laserbeam is scanned by scanning means such as a polygon mirror, agalvanometer mirror or the like, the laser beam is imaged onto therecording surface of a recording material by a scanning lens such as anfθ lens. Characters, an image or the like are thereby recorded onto therecording material.

Generally, curvature of field remains at the scanning lens, and even ifan ideal (i.e., no wave aberration) laser beam is incident on thescanning lens, the beam waist position does not correspond to therecording surface of the recording material. Further, due to reasonssuch as the curvature of the scanning lens being non-uniform, the focalposition (beam waist position) of the laser beam which is scanned by thescanning means and passes through the scanning lens does not alwayscorrespond to the recording surface (so-called "defocus"). Especially ina case in which a semiconductor laser has an astigmatism and the laserbeam irradiated from the semiconductor laser is used as the light beam,there is deviation (so-called astigmatic difference) between the beamwaist position in the main scanning direction of the laser beam and thebeam waist position in the subscanning direction due to the astigmatismof the semiconductor laser and the optical system.

Distortion of the shape of the laser beam illuminated onto the recordingsurface occurs due to the defocus and astigmatic difference, anddeficiencies such as lack of clarity are generated in portions of thecharacters, image or the like recorded on the recording surface. Inrecent years, the demands made on light beam recording devices which canrecord images of larger sizes have increased. Deterioration of imagequality which is caused by the above-mentioned phenomena and whichaccompanies the increase in sizes of recorded images has come to be asignificant problem.

In order to solve this problem, Japanese Patent Application Laid-OpenNo. 3-290610 uses a "cylindrical lens whose focal length can be variedby electric operation". By varying the focal length of the cylindricallens, the beam waist position in the subscanning direction of the laserbeam is varied. Curvature, with respect to the recording surface, of theimage surface in the subscanning direction, which curvature correspondsto the locus of the beam waist position in the subscanning direction ofthe laser beam (hereinafter referred to as "curvature of field") isthereby corrected.

In the above-mentioned publication, the beam waist position is changedby varying the focal length of the cylindrical lens. However, when thefocal length is varied in this way, a drawback arises in that the beamdiameter ω' at the beam waist position of the laser beam exiting fromthe cylindrical lens (hereinafter, "beam waist diameter") fluctuates, asis clear from following formula (1): ##EQU1## wherein ω: beam diameterof the laser beam incident on the cylindrical lens

λ: wavelength of the incident light beam

f: focal length

K: constant.

Accordingly, even if the focal length f of the cylindrical lens isvaried such that the beam waist position of the laser beam alwayscorresponds to the recording surface, the beam waist diameter of thelaser beam varies as the focal length f of the cylindrical lens varies.Therefore, drawbacks still arise such as portions of the characters,images or the like recorded on the recording surface are unclear.

SUMMARY OF THE INVENTION

In view of the aforementioned, an object of the present invention is toprovide a variable focal position optical system in which the beam waistposition of an incident light beam can be moved without the beam waistdiameter thereof being varied. Another object of the present inventionis to provide an light beam scanning apparatus which can illuminate alight beam of a substantially constant beam diameter onto an object tobe illuminated.

In order to achieve the above-described objects, a first aspect of thepresent invention is a variable focal position optical systemcomprising: a variable focal length lens subsystem whose focal length isvariable; and a fixed focal length lens subsystem whose focal length isfixed, wherein the variable focal length lens subsystem and the fixedfocal length lens subsystem are arranged such that a distance between aprincipal point of the variable focal length lens subsystem and aprincipal point of the fixed focal length lens subsystem issubstantially equal to the focal length of the fixed focal length lenssubsystem.

A second aspect of the present invention is a variable focal positionoptical system comprising: a first variable focal length lens subsystemwhich has lens power in a first direction orthogonal to an optical axisand whose focal length is variable; a second variable focal length lenssubsystem which has lens power in a second direction, which isorthogonal to the optical axis and which is different than the firstdirection, and whose focal length is variable; a first fixed focallength lens subsystem which has lens power in the first direction andwhose focal length is fixed; and a second fixed focal length lenssubsystem which has lens power in the second direction and whose focallength is fixed, wherein the first variable focal length lens subsystemand the first fixed focal length lens subsystem are arranged such that adistance between a principal point of the first variable focal lengthlens subsystem and a principal point of the first fixed focal lengthlens subsystem is substantially equal to the focal length of the firstfixed focal length lens subsystem, and the second variable focal lengthlens subsystem and the second fixed focal length lens subsystem arearranged such that a distance between a principal point of the secondvariable focal length lens subsystem and a principal point of the secondfixed focal length lens subsystem is substantially equal to the focallength of the second fixed focal length lens subsystem.

A third aspect of the present invention is a variable focal positionoptical system comprising: a first variable focal length lens subsystemwhich has lens power in a first direction orthogonal to an optical axisand whose focal length is variable; a second variable focal length lenssubsystem which has lens power in a second direction, which isorthogonal to the optical axis and which is different than the firstdirection, and whose focal length is variable; and a fixed focal lengthlens subsystem whose focal length is fixed and which is positioned suchthat the first variable focal length lens subsystem and the secondvariable focal length lens subsystem are positioned at a side of oneprincipal point of the fixed focal length lens subsystem, wherein therespective lens subsystems are arranged such that a difference betweenthe focal length of the fixed focal length lens subsystem and a distancebetween a principal point of the first variable focal length lenssubsystem and a principal point of the fixed focal length lenssubsystem, and a difference between the focal length of the fixed focallength lens subsystem and a distance between a principal point of thesecond variable focal length lens subsystem and a principal point of thefixed focal length lens subsystem, are respectively less than or equalto predetermined values.

A fourth aspect of the present invention is a light beam scanningapparatus comprising: a scanning optical system which scans a light beamirradiated from a light source onto an object to be illuminated; avariable focal position optical system whose focal position is variable;and a control device which controls the focal position of the variablefocal position optical system such that a beam waist position of thelight beam scanned onto the object substantially coincides with asurface of the object. Any one of the variable focal position opticalsystems of the first through the third aspects may be used as thevariable focal position optical system in the fourth aspect.

First, the principles of the present invention will be described. FIGS.1A and 1B illustrate a case in which a parallel light beam of a beamdiameter ω passes through an optical system 10 formed by a variablefocal length lens L_(V) whose focal length can be changed (representedschematically as a single lens in FIG. 1) and a fixed focal length lensL_(F) having a focal length f_(F) (a constant value). The optical system10 is arranged such that the distance between the principal point of thevariable focal length lens L_(V), which principal point is positioned atthe fixed focal length lens L_(F) side, and the principal point of thefixed focal length lens L_(F), which principal point is positioned atthe variable focal length lens L_(V) side, is equal to the focal lengthf_(F) of the fixed focal length lens L_(F).

As illustrated in FIG. 1A, given that f_(V) is the focal length of thevariable focal length lens L, the focal length f_(C) of the opticalsystem 10 formed from the variable focal length lens L_(V) and the fixedfocal length lens L_(F), and the distance S between the rear sideprincipal point of the Fixed focal length lens L_(F) and the focalposition are as expressed by following formulae (2), (3) becaused=f_(F). ##EQU2##

As is clear from formula (2), the focal length f_(C) of the opticalsystem 10 (the focal length of the combination lens formed from thevariable focal length lens L_(V) and the fixed focal length lens L_(F))is equal to the focal length f_(F) of the fixed focal length lens L_(F).Even if the focal length f_(V) of the variable focal length lens L_(V)varies, the focal length f_(C) of the optical system 10 does not vary.In contrast, as is clear from formula (3), the focal position determinedby the distance S varies in accordance with variations in the focallength f_(V) of the variable focal length lens L_(V). This is due to thefact that the position of the rear side principal point of thecombination lens (see FIG. 1A), which position exists at a positionwhich is distanced from the focal position by the focal length f_(C),varies in accordance with variations in the focal length f_(V) of thevariable focal length lens L_(V). With the rear side principal point ofthe fixed focal length lens L_(F) as a reference, the distance S--f_(C)from the position of the rear side principal point of the combinationlens to the rear side principal point of the fixed focal length lensL_(F) can be expressed by the following relation from formulae (2) and(3): ##EQU3##

Let the beam diameter at the focal position (beam waist position) be ω₀' (see FIG. 1A) in a ease in which the focal length of the variablefocal length lens L_(V) is f_(V). Accordingly, the beam diameter ω₁ ' atthe focal position and the amount of variation ΔS in the focal positionwhen the focal length of the variable focal length lens L_(V) increasesby Δf_(V) as illustrated in FIG. 1B are as expressed by followingformulae (4), (5). It is possible to change (move) only the focalposition, i.e., only the beam waist position of the light, by ΔS withoutvarying the beam diameter ω' at the light beam irradiating side of theoptical system 10. ##EQU4## (∴ the beam diameter ω' does not vary)##EQU5##

On the basis of the above discussion, in the first aspect of the presentinvention, the variable focal position optical system includes avariable focal length lens subsystem whose focal length can be changed,and a fixed focal length lens subsystem whose focal length is fixed. Thevariable focal length lens subsystem and the fixed focal length lenssubsystem are arranged such that the distance between the principalpoint of the variable focal length lens subsystem at the fixed focallength lens subsystem side and the principal point of the fixed focallength lens subsystem at the variable focal length lens subsystem sideis substantially equal to the focal length of the fixed focal lengthlens subsystem. In this way, the formulae (4), (5) are substantiallyestablished for the variable focal position optical system relating tothe present invention. Therefore, even if the focal length of thevariable focal length lens subsystem is varied, the focal length of thevariable focal position optical system hardly varies. For a light beamincident on the variable focal position optical system relating to thepresent invention, it is possible to move only the focal positionthereof with hardly any variation in the beam diameter at the focalposition.

The variable focal length lens subsystem and the fixed focal length lenssubsystem may each be structured by a single lens as illustratedschematically in FIG. 1, or may each be structured by a plurality oflenses. In a case in which each lens subsystem is formed by a pluralityof lenses, the positions of the above-described principal pointscorrespond to the positions of the principal points of a singlehypothetical lens which is equivalent to the plurality of lenses, andthe focal length corresponds to the focal length of the singlehypothetical lens. If the pluralities of lenses forming the lenssubsystems are respectively arranged as in the present invention on thebasis of the positions of the principal points and the focal lengths,formulae (4) and (5) can be substantially established.

The lenses forming the variable focal length lens subsystem and thefixed focal length lens subsystem may be lenses having lens power inevery direction orthogonal to the optical axis, such as a lens havingrotation symmetry with respect to the optical axis of the incidentbundle of rays (hereinafter, such a lens will be called a rotationsymmetry lens). Alternatively, the lenses forming the variable focallength lens subsystem and the fixed focal length lens subsystem may belenses which have lens power only in a specific direction, such as acylindrical lens (when a cylindrical lens is used as the lens formingthe variable focal length lens subsystem, only the beam waist positionin the specific direction is varied). However, when the lenses formingthe variable focal length lens subsystem and the fixed focal length lenssubsystem are respectively a lens having lens power in only apredetermined direction, the respective lenses must be arranged suchthat the direction in which the lens forming the variable focal lengthlens subsystem has lens power and the direction in which the lensforming the fixed focal length lens subsystem has lens power coincide.

The focal length of the lens varies in accordance with the refractiveindex of the medium of the lens, the curvatures of the sphericalsurfaces of the lens, and the thickness of the lens. Therefore, a lens,in which at least one of the refractive index, the curvature, and thesurface separation can be changed, may be used as the lens forming thevariable focal length lens subsystem. A lens using an electroopticmaterial such as PLZT and having an electrooptical effect may be used asa lens whose refractive index can be changed. For example, is preferableto use an optical element, such as that which the applicant of thepresent application previously proposed in Japanese Patent ApplicationLaid-Open No. 6-214178, which optical element includes an electroopticmedium having a pair of parallel planes and formed so as to have lenspower in a predetermined direction and exhibiting an electroopticaleffect, and electrodes provided at the pair of parallel planes such thata uniform electric field is applied between the pair of parallel planeswithin the electrooptic medium.

The optical element disclosed in Japanese Patent Application Laid-OpenNo. 1-230017 may be used with the present invention. In thispublication, electrodes are adhered to surfaces of a rectangularelectrooptic medium such that a refractive index distribution isgenerated within the electrooptic medium, and the focal length variesdue to variations in the voltage applied to the electrodes.Alternatively, lenses disclosed in Japanese Patent Application Laid-OpenNos. 62-129814, 62-129815 and 62-25816 may be applied to the presentinvention. The focal lengths of these lenses vary due to variations inthe refractive index of the liquid crystal which serves as anelectrooptic material, which variations are caused by the working of anelectric field. Also applicable to the present invention is the lensdisclosed in Japanese Patent Application Laid-Open No. 62-153933 whosefocal length varies due to variations in the refractive index of anorganic liquid material serving as an electrooptic material, whichvariations are caused by the working of an electric field.

The lens disclosed in Japanese Patent Application Laid-Open No.62-151824, whose focal length varies due to variations in the volume ofa macromolecular gel-like substance which variations are due to electricenergy, can be used as a lens having variable curvature. Further, thistype of lens using a material whose volume varies due to electricalenergy or the like can also be used as a lens in which the thicknessthereof is variable.

The variable focal position optical system relating to the presentinvention may include a plurality of variable focal length lenssubsystems having lens power in respectively different directions. Inthis case, as described in the second aspect of the present invention,it is preferable that the variable focal position optical systemincludes: a first variable focal length lens subsystem which has lenspower in a first predetermined direction and whose focal length isvariable; a second variable focal length lens subsystem which has lenspower in a second predetermined direction, which is different than thefirst predetermined direction, and whose focal length is variable; afirst fixed focal length lens subsystem which has lens power in thefirst predetermined direction and whose focal length is fixed; and asecond fixed focal length lens subsystem which has lens power in thesecond predetermined direction and whose focal length is fixed, whereinthe first variable focal length lens subsystem and the first fixed focallength lens subsystem are arranged such that the distance between aprincipal point of the first variable focal length lens subsystem whichpoint is positioned at a first fixed focal length lens subsystem sideand a principal point of the first fixed focal length lens subsystemwhich point is positioned at a first variable focal length lenssubsystem side is substantially equal to the focal length of the firstfixed focal length lens subsystem, and the second variable focal lengthlens subsystem and the second fixed focal length lens subsystem arearranged such that the distance between a principal point of the secondvariable focal length lens subsystem which point is positioned at asecond fixed focal length lens subsystem side and a principal point ofthe second fixed focal length lens subsystem which point is positionedat a second variable focal length lens subsystem side is substantiallyequal to the focal length of the second fixed focal length lenssubsystem.

When light beams are incident on the variable focal position opticalsystem having the above structure and the focal length of the firstvariable focal length lens subsystem is varied, only the focal positionalong the first predetermined direction (the beam waist position of thelight beam along the first predetermined direction) moves, and the beamwaist position of the light beam along the second predetermineddirection does not change. Similarly, when the focal length of thesecond variable focal length lens subsystem is varied, only the focalposition along the second predetermined direction (the beam waistposition of the light beam along the second predetermined direction)moves, and the focal position of the light beam along the firstpredetermined direction does not move. Even if the focal length ofeither of the variable focal length lens subsystems is varied, there issubstantially no change in the beam waist diameter of the light beam.

Accordingly, by independently and respectively varying the focal lengthof the first variable focal length lens subsystem and the focal lengthof the second variable focal length lens subsystem, the beam waistposition of the light beam along the first predetermined direction andthe beam waist position along the second predetermined direction can bevaried independently without the beam waist diameter of the light beamvarying.

In a case in which the variable focal position optical system isstructured as described above so as to include a plurality of variablefocal length lens subsystems having lens power in respectively differentdirections, the present invention is not limited to a structure such asthat of the second aspect in which a plurality of fixed focal lengthlens subsystems are provided. In accordance with the third aspect of thepresent invention, the variable focal position optical system mayinclude a first variable focal length lens subsystem which has lenspower in a first predetermined direction and whose focal length isvariable; a second variable focal length lens subsystem which has lenspower in a second direction different than the first direction and whosefocal length is variable; and a fixed focal length lens subsystem whichhas lens power in the first predetermined direction and in the secondpredetermined direction and whose focal length is fixed and which isdisposed such that the first variable focal length lens subsystem andthe second variable focal length lens subsystem are positioned at a sideof one principal point of the fixed focal length lens subsystem, whereina difference between a focal length of the fixed focal length lenssubsystem and a distance between the principal point of the firstvariable focal length lens subsystem which point is positioned at afixed focal length lens subsystem side and a principal point of thefixed focal length lens subsystem, and a difference between the focallength of the fixed focal length lens subsystem and a distance betweenthe principal point of the second variable focal length lens subsystemwhich point is positioned at a fixed focal length lens subsystem sideand a principal point of the fixed focal length lens subsystem, arerespectively less than or equal to predetermined values.

As described above, in a case in which the variable focal positionoptical system is structured so as to include a first variable focallength lens subsystem, a second variable focal length lens subsystem,and a single fixed focal length lens subsystem, the beam waist positionof the incident light beam along the first predetermined direction andthe beam waist position along the second predetermined direction can bemoved independently, and the structure of the variable focal positionoptical system can be simplified. In the above structure, it isdifficult to make the distance between the principal point of the firstvariable focal length lens subsystem at the fixed focal length lenssubsystem side and the principal point of the fixed focal length lenssubsystem, and the distance between the principal point of the secondvariable focal length lens subsystem at the fixed focal length lenssubsystem side and the principal point of the fixed focal length lenssubsystem, respectively equal to the focal length of the fixed focallength lens subsystem. When the focal length of each variable focallength lens subsystem is varied, the beam waist diameter may vary.

However, by an arrangement in which the respective differences of theabove distances with respect to the focal length of the fixed focallength lens subsystem are predetermined values or less, even if therespective focal lengths of the first variable focal length lenssubsystem and the second variable focal length lens subsystem are variedin order to move the beam waist position, the variation in the beamdiameter along the first predetermined direction and the variation inthe beam diameter along the second predetermined direction canrespectively be kept small.

In any of the inventions of the first through the third aspects, in acase in which a substantially parallel beam is incident on the variablefocal position optical system or in a case in which a light beam ofsubstantially parallel light rays emerges from the variable focalposition optical system, each lens subsystem is disposed on the opticalpath of the light beam such that the fixed focal length lens subsystemas viewed from the variable focal length lens subsystem is positioned atat least one of a point of convergence side and a point of divergenceside, i.e., at the side at which the substantially parallel beam isincident or emerges and the side opposite thereto (see FIGS. 9A, 9B,10A, 10B). In this way, formulae (4), (5) are substantially establishedfor the variable focal position optical system related to the presentinvention.

Accordingly, in a case which the respective lens subsystems are arrangedsuch that the substantially parallel beam is incident on the variablefocal position optical system, variations in the beam waist diameter atthe point of convergence positioned at the side of the fixed focallength lens subsystem which side is opposite the side at which thevariable focal length lens subsystem is disposed, when the focal lengthof the variable focal length lens subsystem is varied, can besuppressed. The same effects can be achieved even if the parallel lightdiverges slightly. Further, as illustrated in FIGS. 9A, 9B, 10A, 10B, ina case in which the respective lens subsystems are arranged so thatdivergent light from the point of divergence is incident on the variablefocal position optical system, when the focal length of the variablefocal length lens subsystem is varied, the beam diameter and degree ofparallelism of the substantially parallel light beam exiting from thevariable focal position optical system can be varied such that theposition of the point of divergence hypothetically moves with littlevariation in the beam waist diameter at the point of divergence.

In the fourth aspect of the present invention, the variable focalposition optical system of any of the first through the third aspects isdisposed on the optical path of a light beam, and the focal length ofthe variable focal length lens subsystem of the variable focal positionoptical system is controlled such that the beam waist position of thelight beam scanned onto the object to be illuminated substantiallycoincides with the surface to be illuminated. In the inventions of thefirst through the third aspects described above, variations in the beamwaist diameter of a light beam in a case in which the focal length ofthe variable focal length lens subsystem is varied and the beam waistposition of the light beam is varied, can be controlled. Accordingly, bydisposing the variable focal position optical system of any of the firstthrough the third aspects on the optical path of the light beam, a lightbeam of substantially uniform beam diameter can be illuminated onto theobject to be illuminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an explanatory view illustrating a structure of an opticalsystem, for explaining operation of the present invention.

FIG. 1B is an explanatory view illustrating a structure of an opticalsystem, for explaining operation of the present invention.

FIG. 2 is a perspective view illustrating a schematic structure of alaser beam recording device.

FIG. 3A is a plan view illustrating a meridional direction schematicstructure of a laser beam irradiating device relating to a firstembodiment.

FIG. 3B is a plan view illustrating a sagittal direction schematicstructure of the laser beam irradiating device relating to the firstembodiment.

FIG. 4 is a perspective view illustrating an example of an opticalelement which serves as a variable focal length lens subsystem.

FIG. 5 is a block view illustrating a schematic structure of a focalposition control device.

FIG. 6 is a conceptual view for explaining image surfaces, astigmaticdifference and offset in the meridional and sagittal directions.

FIG. 7A is a plan view illustrating a meridional direction schematicstructure of a laser beam irradiating device relating to a secondembodiment.

FIG. 7B is a plan view illustrating a sagittal direction schematicstructure of the laser beam irradiating device relating to the secondembodiment.

FIG. 8A is a perspective view illustrating another example of an opticalelement.

FIG. 8B is a perspective view illustrating still another example of anoptical element.

FIG. 8C is a perspective view illustrating yet another example of anoptical element.

FIG. 9A is a plan view illustrating a meridional direction schematicstructure of a variant example of the laser beam irradiating devicerelating to the first embodiment.

FIG. 9B is a plan view illustrating a sagittal direction schematicstructure of a variant example of the laser beam irradiating devicerelating to the first embodiment.

FIG. 10A is a plan view illustrating a meridional direction schematicstructure of a variant example of the laser beam irradiating devicerelating to the second embodiment.

FIG. 10B is a plan view illustrating a sagittal direction schematicstructure of a variant example of the laser beam irradiating devicerelating to the second embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 illustrates a laser beam recording device 30 relating to a firstembodiment. The laser beam recording device 30 includes a laser beamirradiating device 32 which irradiates a laser beam of substantiallyparallel rays. The laser beam irradiating device 32 will be described inmore detail later. A cylindrical lens 31 for correcting the tilt of thesurface of a polygon mirror and a polygon mirror 46 are disposed at theirradiating side of the laser beam irradiating device 32. The rotationof the polygon mirror 46 is controlled by a drive control circuit 47.

A semiconductor laser for synchronizing 48 which irradiates a laser beamfor synchronizing is disposed in a vicinity of the polygon mirror 46. Acollimator lens 50 is disposed at the laser beam irradiating side of thesemiconductor laser for synchronizing 48. The laser beam irradiated fromthe semiconductor laser for synchronizing 48 is made into a parallelbundle of rays at the collimator lens 50, and is then made incident ontothe polygon mirror 46. The laser beam for recording, which is irradiatedfrom the laser beam irradiating device 32, and the laser beam forsynchronizing, which exits from the collimator lens 50, are illuminatedonto substantially the same region of the polygon mirror 46, and aredeflected in the same way in the main scanning direction as the polygonmirror 46 rotates, and are made incident on a fθ lens 52 disposed at thelaser beam exiting side of the polygon mirror 46.

A cylindrical lens 33 for correcting the tilt of the surface of apolygon mirror and a drum 54 are disposed along the scanning directionin that order on the optical path of the laser beam for recording whichhas passed through the fθ lens 52. An unillustrated photosensitivematerial is wound on the drum 54. The laser beam for recording isilluminated onto the recording surface of the photosensitive material,and is scanned over the range illustrated by the broken line in FIG. 2due to deflection by the polygon mirror 46. Further, due to the rotationof the drum 54, the position at which the laser beam is illuminated ontothe photosensitive material moves in the subscanning direction as well,so that an image is recorded onto the photosensitive material.

Hereinafter, the direction corresponding to the main scanning directionof the laser beam which passes through the scanning optical system, suchas the polygon mirror 46, the fθ lens 52, and the like, and which isilluminated onto the drum 54 is called the meridional direction, and thedirection corresponding to the subscanning direction thereof is calledthe sagittal direction. Further, the plane which includes the directionof the optical axis of the laser beam and the meridional direction iscalled the meridional plane, and the plane which includes the opticalaxis direction and the sagittal direction is called the sagittal plane.

A reflecting mirror 56 is disposed along the scanning direction of thelaser beam for synchronizing on the optical path thereof. A linearencoder 58 is disposed at the laser beam exiting side of the reflectingmirror 56. The laser beam for synchronizing is deflected by the polygonmirror 46 so as to scan the range illustrated by the dot-chain line inFIG. 2. The linear encoder 58 is formed such that a plurality ofband-shaped, non-transparent portions having equal widths are formed atpredetermined intervals on a transparent plate member. An unillustratedphotoelectric converter is disposed at the laser beam exiting side ofthe linear encoder 58. When the laser beam is scanned on the linearencoder 58, the photoelectric converter receives the laser beam whichhas passed through the transparent portions, and a pulse signal isoutputted from the photoelectric converter.

The laser beam irradiating device 32 will now be described. Asillustrated in FIGS. 3A and 3B, the laser beam irradiating device 32includes a semiconductor laser 34. In FIGS. 3A and 3B, the arrow Mindicates the meridional direction, and the arrow S indicates thesagittal direction. The semiconductor laser 34 is modulated, by anunillustrated modulating device, in accordance with the image to berecorded and at a timing synchronous with the pulse signal, and a laserbeam corresponding to the image to be recorded is irradiated.

A collimator lens 36 is disposed at the laser beam irradiating side ofthe semiconductor laser 34. The laser beam irradiated from thesemiconductor laser 34 is made into substantially parallel light at thecollimator lens 36, and exits therefrom. The variable focal positionoptical system relating to the present invention is disposed at thelaser beam exiting side of the collimator lens 36 and includes, in thefollowing order, an optical element 38 and a half-wave plate 39 servingas a variable focal length lens subsystem; an optical element 40 servingas a variable focal length lens subsystem; a cylindrical lens 41 servingas a fixed focal length lens subsystem; and a cylindrical lens 42serving as a fixed focal length lens subsystem.

As illustrated in FIG. 4, the optical element 38 is provided with aelectrooptic medium 38A which is formed as if a cylinder was cut along aplane parallel to the axis of the cylinder. Namely, the electroopticmedium 38A is formed such that a pair of parallel planes correspondingto the upper surface and lower surface thereof are respectively enclosedby a circular arc and a chord. The electrooptic medium 38A is formed byPLZT serving as an electrooptic material, and exhibits an electroopticaleffect. Further, electrodes 38B, 38C are each deposited over the entiresurface of one of the pair of parallel planes of the electrooptic medium38A.

In the optical element 38 illustrated in FIGS. 3A and 3B, the directionin which the electrooptic medium 38A has lens power is disposed so as torun along the sagittal plane of the incident laser beam. The focallength f_(VS) of the electrooptic medium 38A in the sagittal directionis determined by the following formula (6) in which n₀ is the refractiveindex of the electrooptic medium 38A and r is the radius of curvature ofthe circular arc shaped portions of the parallel planes.

    f.sub.VS =r÷(n.sub.0 -1)                               (6)

Accordingly, the laser beam incident on the optical element 38 isrefracted only within the sagittal plane of the laser beam and exitsfrom the optical element 38. However, in a case in which voltage is notapplied between the electrodes 38B, 38C, the substantially parallellaser beam incident on the electrooptic medium 38A is refracted suchthat the beam waist in the sagittal direction is formed at the positionof the focal length f_(VS) in the sagittal direction, and the laser beamexits. Here, the polarization direction corresponds to the direction ofarrow M in FIG. 3.

Further, when voltage is applied between the electrodes 38B, 38C, auniform electric field is applied between the electrodes 38B, 38C, i.e.,between the parallel planes of the inner portion of the electroopticmedium 38A. When a voltage V is applied between the electrodes 38B, 38C,given that the distance between the electrodes 38B, 38C is d, thestrength E of the electric field between the electrodes 38B, 38C is:

    E=V÷d                                                  (7)

The direction of the magnetic field is the direction of arrow A in FIG.4. Due to this magnetic field, when the secondary electroopticcoefficient (Kerr coefficient) of the electrooptic material forming theelectrooptic medium 38A is R₃₃, the refractive index in the meridionaldirection of the electrooptic medium 38A is varied from the refractiveindex n₀ to the refractive index n expressed by the following formula(8). ##EQU6##

In accordance with formula (8), the refractive index of the electroopticmedium 38A varies proportionately to the square of the strength E of theelectric field, i.e., varies proportionately to the square of thevoltage V applied between the electrodes 38B, 38C. Further, because theelectric field at this time is a uniform electric field, the refractiveindex in the meridional direction of the electrooptic medium 38A alsovaries uniformly. Due to the variation in refractive index, the focallength f_(VS) of the electrooptic medium 38A in the sagittal directionwhen the voltage V is applied between the electrodes 38B, 38C varies inaccordance with following formula (9). ##EQU7##

In this way, at the optical element 38, when no voltage is appliedbetween the electrodes, 38B, 38C, the electrooptic medium 38A has apredetermined lens power in the sagittal direction in FIG. 4. Therefore,as compared with the optical element disclosed in Japanese PatentApplication Laid-Open No. 1-230017, when the distance between theoptical element and the beam waist position of the light beam in apredetermined direction is short on the whole, it is possible to applyless voltage between the electrodes 38B, 38C, and there is no need tocreate a refractive index distribution which generates lens power at theinterior of the electrooptic medium 38A. Therefore, the effect of theaberration caused by the difference between the generated refractiveindex distribution and an ideal refractive index distribution can beeliminated, and even in a case in which a laser beam is refracted byhigh lens power, the beam waist diameter can be prevented from becominglarge.

The half-wave plate 39 and the optical element 40 are disposed in thatorder at the laser beam exiting side of the optical element 38. Thelaser beam exiting from the optical element 38 passes through thehalf-wave plate 39, and the polarization direction thereof is therebyrotated 90 degrees so as to correspond to the direction of arrow S inFIG. 3, and the laser beam is incident on the optical element 40. Theoptical element 40 is structured in the same way as the optical element38. The optical element 40 is provided with an electrooptic medium 40Aformed as if a cylinder had been cut along a plane parallel to the axisof the cylinder. Electrodes 40B, 40C are each formed over the entiresurface of one of the parallel planes of the electrooptic medium 40A.

At the optical element 40, the direction in which the electroopticmedium 40A has lens power is disposed along the meridional plane.Accordingly, the focal length f_(VM) of the optical element 40 in themeridional direction varies in proportion to the square of the voltageapplied between the electrodes 40B, 40C. The laser beam incident on theoptical element 40 is refracted only within the meridional plane of thelaser beam, and exits from the optical element 40.

The cylindrical lens 41 disposed at the laser beam exiting side of theoptical element 40 is disposed such that, in the same way as the opticalelement 38, the direction in which the optical element 40 has lens powercorresponds to the sagittal direction. The cylindrical lens 41 isdisposed such that the distance D_(S) (see FIG. 3B) between theprincipal point of the cylindrical lens 41 at the laser beam incidentside (hereinafter, "front side principal point") and the principal pointof the optical element 38 at the laser beam exiting side (hereinafter,"rear side principal point") is equal to the focal length f_(FS) of thecylindrical lens 41 in the sagittal direction.

The cylindrical lens 42 disposed at the laser beam exiting side of thecylindrical lens 41 is disposed such that the direction in which thecylindrical lens 42 has lens power corresponds to the meridionaldirection, in the same way as the optical element 40. The cylindricallens 42 is disposed such that the distance D_(M) (see FIG. 3A) betweenthe front side principal point of the cylindrical lens 42 and the rearside principal point of the optical element 40 is equal to the focallength f_(FM) of the cylindrical lens 42 in the meridional direction.

The lens 43 is disposed at the laser beam exiting side of thecylindrical lens 42. The beam waist of the laser beam which exits fromthe cylindrical lens 42 is formed temporarily, and after the beamdiameter expands, the laser beam is incident on the lens 43. At the lens43, the laser beam is made into a parallel bundle of rays, exits fromthe laser beam irradiating device 32, and is incident on the polygonmirror 46. A spatial filer 44 is disposed at the position at which thebeam waist of the laser beam between the cylindrical lens 42 and thelens 43 is formed.

The spatial filter 44 is formed by pinholes being formed in anon-transparent plate member, and cuts the scattered light component ofthe laser beam. It is well-known that the scattered light componentarising when a light beam passes through PLZT cannot be ignored. Thescattered light component of the laser beam is cut by the spatial filter44.

Turning again to FIG. 2, the drive control circuit 47 is connected to afocal position control device 62. The drive control circuit 47 outputsto the focal position control device 62 a signal representing theillumination position of the laser beam along the main scanningdirection. The focal position control device 62 is structured asillustrated in FIG. 5. Namely, the signal inputted from the drivecontrol circuit 47 is inputted to a control section 68. The controlsection 68 is formed by a microcomputer in which a CPU, RAM, ROM and thelike are connected together by busses such as data busses, addressbusses or the like. The control section 68 is connected to a storagedevice 70.

In the laser beam recording device 30 of the first embodiment, themeridional direction and sagittal direction beam waist positions of thelaser beam when the laser beam is illuminated onto the photosensitivematerial wound on the drum are successively measured while theillumination position is moved along the main scanning direction. Asillustrated in FIG. 6, data representing an image surface 100 in themeridional direction and an image surface 102 in the sagittal directionare obtained in advance.

The deviation ΔZ_(S) of the image surface 102 in the sagittal directionwith respect to a reference surface (the recording surface of thephotosensitive material) at each position along the main scanningdirection is determined. Correction data for correcting the deviationΔZ_(S) by the optical element 38 is calculated. The correction data andthe respective positions are set in correspondence and stored in thestoring device 70 as a first table. Further, the deviation ΔZ_(M) of theimage surface 100 in the meridional direction with respect to thereference surface (the recording surface of the photosensitive material)at each position along the main scanning direction is determined.Correction data for correcting the deviation ΔZ_(M) by the opticalelement 40 is calculated. The correction data and the respectivepositions are set in correspondence and stored in the storing device 70as a second table.

Given that the lateral magnification of the optical system (includingthe lens 43 of the laser beam irradiating device 32 and the scanninglens 52) through which the laser beam irradiated from the variable focalposition optical system passes is β, when the beam waist position of thelaser beam which position is formed at the laser beam exiting side ofthe variable focal position optical system moves ΔS, the amount ofmovement ΔS' of the beam waist position of the laser beam exiting fromthe scanning lens 52 is as follows:

    ΔS'=ΔS·β.sup.2                   (10)

The above-mentioned correction data is calculated on the basis of thedeviation ΔZ_(S) or ΔZ_(M), and on the basis of formulae (5), (8), (9)and (10). Given that the beam waist diameter of the laser beam exitingfrom the variable focal position optical system is ω', the beam waistdiameter ω" of the laser beam exiting from the scanning lens 52 is:

    ω"=ω'·β                          (11)

Digital/analog converters (D/A converters) 72A, 72B are connected to theoutput side of the control section 68. On the basis of the signal fromthe drive control device 47, the control section 68 determines theillumination position of the laser beam along the main scanningdirection. The correction data corresponding to this illuminationposition is fetched from the first table stored in the storage device 70and is outputted to the D/A converter 72A. Further, the correction datacorresponding to the illumination position is fetched from the secondtable and is outputted to the D/A converter 72B.

An amplifier circuit 74A is connected to the output end of the D/Aconverter 72A, and the electrode 38B of the optical element 38 isconnected to the amplifier circuit 74A. The electrode 38C is grounded.The correction data outputted from the control section 68 is convertedat the D/A converter 72A into an analog signal whose voltage levelcorresponds to the value of the correction data. The signal is amplifiedand boosted at the amplifier circuit 74A, and is supplied to theelectrode 38B. Accordingly, voltage of a level corresponding to themagnitude of the correction data outputted from the control section 68is applied between the electrodes 38B, 38C. Similarly to the abovestructure, an amplifier circuit 74B is connected to the output end ofthe D/A converter 72B, and the electrode 40B is connected to theamplifier circuit 74B. The electrode 40C is grounded. In the same way asdescribed above, voltage is applied between the electrodes 40B, 40C.

Next, operation of the first embodiment will be described. The laserbeam irradiated from the semiconductor laser 34 successively passesthrough the collimator lens 36, the optical element 38, the half-waveplate 39, the optical element 40, the cylindrical lenses 41, 42, thespatial filter 44, and the lens 43, and is irradiated from the laserbeam irradiating device 32.

The laser beam for recording which is irradiated from the laser beamirradiating device 32 is deflected along the main scanning direction bythe polygon mirror 46 together with the laser beam for synchronizing,and is scanned on the recording surface of the photosensitive material.Further, the laser beam for synchronizing is scanned onto the lightreceiving surface of the linear encoder 58. The laser beam which haspassed through the transparent portions of the linear encoder 58 due tothis scanning is received at the unillustrated electrooptic converter,and a pulse signal is inputted from the electrooptic converter to thedrive control circuit 47. At the drive control circuit 47, a signalrepresenting the illumination position of the laser beam along the mainscanning direction is outputted to the control section 68 of the focalposition control circuit 62.

On the basis of the signal inputted to the control section 68,corresponding correction data is successively fetched from the firsttable and the second table in accordance with the movement of theillumination position of the laser beam along the main scanningdirection. The correction data is outputted to the D/A converters 72A,72B. In this way, voltage of a level corresponding to the correctiondata fetched from the first table is applied between the electrodes 38B,38C of the optical element 38 via the D/A converter 72A and theamplifier circuit 74A. Further, voltage of a level corresponding to thecorrection data fetched from the second table is applied between theelectrodes 40B, 40C of the optical element 40 via the D/A converter 72Band the amplifier circuit 74B.

When voltage is applied between the electrodes 38B, 38C of the opticalelement 38, a uniform electric field corresponding to the value of theapplied voltage is applied to the electrooptic medium 38A of the opticalelement 30. The sign of the secondary electrooptic coefficient (Kerrcoefficient) R₃₃ of the PLZT forming the electrooptic medium 38A of theoptical element 38 is positive. Accordingly, as is clear from formula(8), due to the application of the voltage, the refractive index in thedirection in which the electrooptic element 38A has lens power decreasesin proportion to the square of the applied voltage, and the lens powerdecreases (the focal length f_(VS) becomes longer).

At the laser beam irradiating device 32, the laser beam irradiated fromthe semiconductor laser 34 is refracted within the sagittal plane by theoptical element 38 and the cylindrical lens 41. As described above, theoptical element 38 and the cylindrical lens 41 are disposed such thatthe distance D_(S) between the rear side principal point of the opticalelement 38 and the front side principal point of the cylindrical lens 41is equal to the focal length f_(FS) of the cylindrical lens 41 in thesagittal direction. Therefore, in the optical system formed by theoptical element 38 and the cylindrical lens 41, the focal length f_(CS)in the sagittal direction, the focal position in the sagittal direction,the beam waist diameter ωS' in the sagittal direction, and the amount ofvariation ΔS_(S) in the focal position (beam waist position) in thesagittal direction are as determined by above formulae (2) through (5)respectively.

In this way, the beam waist position in the sagittal direction of thelaser beam, which is irradiated from the laser beam irradiating device32 and illuminated onto the drum 54, moves in accordance with formulae(5) and (10) in accordance with the focal length f_(VS) of the opticalelement 38 which becomes longer as the voltage applied between theelectrodes 38B, 38C of the optical element 38 increases. As is clearfrom formula (4), even if the focal length f_(VS) of the optical element38 varies, the beam waist diameter ω_(S) ' in the sagittal direction ofthe laser beam emerging from the variable focal position optical systemdoes not vary. Therefore, as is clear from formula (11), the beam waistdiameter ω_(S) " in the sagittal direction of the laser beam illuminatedonto the drum 54 is constant.

As described above, at the focal position control circuit 62, correctiondata is fetched from the first table on the basis of the signaloutputted from the drive control circuit 47. The fetched correction datais converted into an analog signal which is amplified, and voltage isapplied between the electrodes 38B, 38C of the optical element 38.Therefore, the deviation ΔZ_(S) of the laser beam illuminated onto thedrum 54 is substantially zero from the one end to the other end in themain scanning direction, and the image surface 102 in the sagittaldirection corresponds to the recording surface of the photosensitivematerial wound on the drum 54.

When voltage is applied between the electrodes 40B, 40C of the opticalelement 40 as well, the refractive index in the direction in which theelectrooptic medium 40A has lens power decreases in proportion to thesquare of the applied voltage, and the lens power decreases (the focallength f_(VM) becomes longer). The optical element 40 and thecylindrical lens 42 of the laser beam irradiating device 32 are alsodisposed such that the distance D_(M) between the rear side principalpoint of the optical element 40 and the front side principal point ofthe cylindrical lens 42 is equal to the focal length f_(FM) of thecylindrical lens 42 in the meridional direction. Therefore, in theoptical system formed by the optical element 40 and the cylindrical lens42, the focal length f_(CM) in the meridional direction, the focalposition, the beam waist diameter ω_(M) ' and the amount of changeΔS_(M) in the focal position (beam waist position) are as determined byabove formulae (2) through (5) respectively.

In this way, the beam waist position in the meridional direction of thelaser beam, which is irradiated from the laser beam irradiating device32 and illuminated onto the drum 54, moves in accordance with formulae(5) and (10) in accordance with the focal length f_(VM) of the opticalelement 40 which becomes longer as the voltage applied between theelectrodes 40B, 40C of the optical element 40 increases. As is clearfrom formula (4), even if the focal length f_(VM) of the optical element40 varies, the beam waist diameter ω_(M) ' in the meridional directionof the laser beam emerging from the variable focal position opticalsystem does not vary. Therefore, as is clear from formula (11), the beamwaist diameter ω_(M) " in the meridional direction of the laser beamilluminated onto the drum 54 is constant.

As described above, at the focal position control circuit 62, correctiondata is fetched from the second table on the basis of the signaloutputted from the drive control circuit 47. The fetched correction datais converted into an analog signal which is amplified, and voltage isapplied between the electrodes 40B, 40C of the optical element 40.Therefore, the deviation ΔZ_(M) of the laser beam illuminated onto thedrum 54 is substantially zero from the one end to the other end in themain scanning direction, and the image surface 100 in the meridionaldirection corresponds to the recording surface of the photosensitivematerial wound on the drum 54.

In this way, the beam waist positions of the laser beam incident on thefθ lens 52 are moved by variations in the focal lengths of the opticalelements 38, 40. Due to this movement, the beam waist positions of thelaser beam in the meridional direction and the sagittal direction fromone end to the other end in the scanning direction always correspond tothe recording surface of the photosensitive material. Further, the beamwaist diameter of the laser beam does not vary, regardless of variationsin the focal lengths of the optical elements 38, 40. Accordingly, ahigh-quality image can be obtained without drawbacks such as portions ofthe image recorded on the photosensitive material being unclear.

The above description includes an example in which the distance D_(S) ismade equal to the focal length f_(FS) of the cylindrical lens 41 and thedistance D_(M) is made equal to the focal length f_(FM) of thecylindrical lens 42. However, the present invention is not limited tothe same. If the distances and focal lengths are close values, even ifthey are not exactly equal, variations in the beam waist positions ω_(S)", ω_(M) " accompanying variations in the focal lengths f_(FS), f_(FM)can be suppressed.

In the above explanation, the deviations ΔZ_(S), ΔZ_(M) of the imagesurfaces are measured in advance, correction data is determined, and thefocal length of the variable focal length lens subsystem is varied onthe basis of the correction data. However, the beam waist positionfluctuates in accordance with variations in characteristics andvariations in the positions of the respective lenses and the like whichare due to variations in the ambient temperature. Therefore, the ambienttemperature may be measured by a detecting means such as a temperaturesensor, and the focal length of the variable focal length lens subsystem(the voltage applied to the optical element 38 in the presentembodiment) may be corrected.

A second embodiment of the present invention will be describedhereinafter. Other than the laser beam irradiating device 32, the secondembodiment is structured in the same way as the first embodiment.Therefore, for portions of the second embodiment other than the laserbeam irradiating device 32, the same reference numerals are used, anddescription thereof is omitted.

As illustrated in FIG. 7, in the laser beam irradiating device 32relating to the second embodiment, a single rotation symmetry lens 80,which serves as a fixed focal length lens subsystem, is used in place ofthe cylindrical lenses 41, 42. In this way, the structure of thevariable focal position optical system can be simplified, thereliability of the device can be improved, and the cost thereof can bereduced. In this structure, the position of the rear side principalpoint of the optical element 38 and the position of the rear sideprincipal point of the optical element 40 cannot be made to coincide.Therefore, it is impossible to dispose the optical elements 38, 40 andthe lens 80 such that the distance D_(S) between the rear side principalpoint of the optical element 38 and the front side principal point ofthe lens 80, and the distance D_(M) between the rear side principalpoint of the optical element 40 and the front side principal point ofthe lens 80 are respectively equal to the focal length f_(F) of the lens80.

As a result, in the second embodiment, the lens 80 is disposed at leastsuch that the following formula (12) is satisfied and such that thedifference between the distance D_(S) and the focal length f_(F) and thedifference between the distance D_(M) and the focal length f_(F) arerespectively less than or equal to predetermined values.

    MIN[D.sub.S,D.sub.M ]≦f.sub.F ≦MAX[D.sub.S,D.sub.M ](12)

In the second embodiment, because D_(M) <D_(S) as shown in FIG. 7,formula (12) becomes:

    D.sub.M ≦f.sub.F ≦D.sub.S

In order to make the difference between the distance D_(S) and the focallength f_(F) and the difference between the distance D_(M) and the focallength f_(F) small respectively, first, it is essential to make thedistance between the optical element 38 and the optical element 40 assmall as possible. As for specifically what values the differencesbetween the focal length f_(F) and the distances D_(S), D_(M) are to beset to, the lens 80 can be disposed such that, for example, thedifference between the distance D_(S) and the focal length f_(F) and thedifference between the distance D_(M) and the focal length f_(F) areequal. In this case, when the focal length f_(VS) of the optical element38 and the focal length f_(VM) of the optical element 40 arerespectively varied, large variations in one of the sagittal directionbeam waist diameter ω_(S) " and the meridional direction beam waistdiameter ω_(M) " are suppressed.

When the laser beam is scanned and the image is recorded, in a ease inwhich, for example, the recording density in the main scanning directionand the recording density in the subscanning direction are different,the ratio of the decrease in image quality to the variation in beamwaist diameter differs for the main scanning direction and thesubscanning direction, and the allowance with respect to the beam waistdiameter of the illuminated laser beam differs for the main scanningdirection and the subscanning direction. In such a ease, it ispreferable to arrange the optical elements 38, 40 and the lens 80 suchthat the distance D corresponding to the direction in which theallowance is smaller becomes the value which is closest to the focallength f_(F) compared with the other distance D, or such that thedistance D corresponding to the direction in which the allowance issmaller is equal to the focal length f_(F).

For example, in a case in which the recording density in the subscanningdirection is low and the allowance for variations in the beam waistdiameter is small, it is preferable to arrange the optical elements 38,40 and the lens 80 either such that the distance D_(S) corresponding tothe subscanning direction (the sagittal direction in the presentembodiment) becomes the value closest to the focal length f_(F) ascompared with the distance D_(M) or such that the distance D_(S) isequal to the focal length f_(F). In this way, it is difficult forirregularities in density or the like due to variations in the beamwaist diameter to occur. Therefore, deterioration in image quality canbe suppressed.

Lenses which are preferable for the variable focal length lens subsystemrelating to the present invention, which lenses respectively comprise anelectrooptic medium equipped with a pair of parallel planes and formedso as to have lens power in a predetermined direction and exhibiting anelectrooptical effect, and electrodes which are provided respectively atthe pair of parallel planes such that a uniform electric field isapplied between the pair of parallel planes within the electroopticmedium, are not limited to the optical element 38 illustrated in FIG. 4.For example, in the optical element 20 illustrated in FIG. 8A, theconfiguration of an electrooptic medium 20A is a cylinder in which theparallel planes are circular, such as a so-called "rodless". Theelectrooptic medium 20A has lens power in the direction of arrow Y inFIG. 8. Accordingly, when a light beam is incident along the directionof arrow Z in FIG. 8, the light beam is refracted within a predeterminedplane including the arrow Y and the arrow Z. Further, electrodes 20B,20C are provided over the entire surfaces of the axial direction endsurfaces of the cylinder of the electrooptic medium 20. Accordingly,when voltage is applied between the electrodes 20B, 20C, a uniformelectric field is applied between the axial direction end surfaces. Byvarying the magnitude of the voltage, the converging position of thelight beam within the above-mentioned predetermined plane can bechanged.

In an optical element 22 illustrated in FIG. 8B, the configuration of anelectrooptic medium 22A is an elliptic cylinder in which the parallelplanes are elliptic. In the same way as the electrooptic medium 20A ofthe optical element 20, the electrooptic medium 22A has lens power inthe direction of arrow Y in FIG. 8. Further, electrodes 22B, 22C areprovided over the entire surfaces of the axial direction end surfaces ofthe elliptic cylinder. Accordingly, when voltage is applied between theelectrodes 22B, 22C, a uniform electric field is applied between theaxial direction end surfaces. By varying the magnitude of the voltage,the converging position of the light beam within the predetermined planecan be changed.

In an optical element 24 illustrated in FIG. 8C, the configuration ofeach parallel plane of an electrooptic medium 24A is a shape such as thecross-section of a Fresnel lens, and the electrooptic medium 24A is apillar having such parallel planes as axial direction end surfaces. As aresult, the electrooptic medium 24A has lens power in the direction ofarrow Y in FIG. 8 in the same way as described above. Electrodes 24B,24C are provided over the entire surfaces of the axial direction endsurfaces. Accordingly, when voltage is applied between the electrodes24B, 24C, a uniform electric field is applied between the axialdirection end surfaces. By varying the magnitude of the voltage, theconverging position of the light beam within the predetermined plane canbe changed.

In the above explanation, an example was described in which asemiconductor laser is used as the light source, and a laser beamirradiated from the semiconductor laser is used as the light beam.However, the light source for irradiating a laser beam may be a gaslaser such as a He-Ne laser, an Ar laser, or the like. The laser beamirradiated from the gas laser may be modulated by a modulator such as anAOM, an EOM or the like. Further, an LED may be used as the lightsource, and the light beam irradiated from the LED may be used as thelight beam in the present invention.

Further, the above description includes an example in which a devicewhich illuminates a laser beam directly onto the photosensitive materialand records an image is used as the laser beam recording device.However, the present invention is also applicable to, for example, alaser beam recording device which serves as a light source when an imageis recorded by an electrophotographic method such that an electrostaticlatent image is recorded on a photoreceptor drum which has been chargedin advance.

In the above explanation, the laser beam recording device is used as alight beam scanning apparatus. However, the present invention is alsoapplicable to a light beam reading apparatus which reads an image or thelike by using a light beam such as a laser beam.

As described above, the first aspect of the present invention is avariable focal position optical system including a variable focal lengthlens subsystem and a fixed focal length lens subsystem, wherein thevariable focal length lens subsystem and the fixed focal length lenssubsystem are arranged such that a distance between a principal point ofthe variable focal length lens subsystem and a principal point of thefixed focal length lens subsystem is substantially equal to the focallength of the fixed focal length lens subsystem. Therefore, a superioreffect is achieved in that it is possible to move only the focalposition with hardly any variation in the beam diameter at the focalposition of the incident light beam.

The second aspect of the present invention is a variable focal positionoptical system including first and second variable focal length lenssubsystems and first and second fixed focal length lens subsystems,wherein the first variable focal length lens subsystem and the firstfixed focal length lens subsystem are arranged such that a distancebetween a principal point of the first variable focal length lenssubsystem and a principal point of the first fixed focal length lenssubsystem is substantially equal to the focal length of the first fixedfocal length lens subsystem, and the second variable focal length lenssubsystem and the second fixed focal length lens subsystem are arrangedsuch that a distance between a principal point of the second variablefocal length lens subsystem and a principal point of the second fixedfocal length lens subsystem is substantially equal to the focal lengthof the second fixed focal length lens subsystem. Therefore, a superioreffect can be achieved in that the beam waist positions along a firstpredetermined direction and a second predetermined direction of thelight beam can be moved independently without variation in the beamwaist diameter of the light beam.

The third aspect of the present invention is a variable focal positionoptical system including first and second variable focal length lenssubsystems and a single fixed focal length lens subsystem wherein therespective lens subsystems are arranged such that a difference betweenthe focal length of the fixed focal length lens subsystem and a distancebetween a principal point of the first variable focal length lenssubsystem and a principal point of the fixed focal length lenssubsystem, and a difference between the focal length of the fixed focallength lens subsystem and a distance between a principal point of thesecond variable focal length lens subsystem and a principal point of thefixed focal length lens subsystem, are respectively less than or equalto predetermined values. Therefore, excellent effects can be achieved inthat the beam waist positions along a first predetermined direction anda second predetermined direction of the incident light beam can be movedindependently, the structure of the variable focal position opticalsystem can be simplified, and variations in the beam diameter along thefirst predetermined direction and the second predetermined direction canrespectively be kept small.

In the fourth aspect of the present invention, the variable focalposition optical system of any of the first through the third aspects isdisposed on the optical path of a light beam. The focal length of thevariable focal length lens subsystem of the variable focal positionoptical system is controlled such that the beam waist position of thelight beam scanned onto an object to be illuminated substantiallycorresponds to the surface to be illuminated. Therefore, an excellenteffect is achieved in that a light beam of a substantially constant beamdiameter can be illuminated onto the object to be illuminated.

What is claimed is:
 1. A variable focal position optical systemcomprising:a variable focal length lens subsystem whose focal length isvariable; and a fixed focal length lens subsystem whose focal length isfixed, said variable focal length lens subsystem and said fixed focallength lens subsystem being arranged such that a distance between aprincipal point of said variable focal length lens subsystem and aprincipal point of said fixed focal length lens subsystem issubstantially equal to the focal length of said fixed focal length lenssubsystem.
 2. A variable focal position optical system according toclaim 1, wherein said fixed focal length lens subsystem and saidvariable focal length lens subsystem are anamorphic lens systems.
 3. Avariable focal position optical system according to claim 1, whereinsaid variable focal length lens subsystem includes an electroopticmedium having an electrooptical effect.
 4. A variable focal positionoptical system claimed in claim 1, wherein a focal length of the opticalsystem remains constant when the focal length of the variable focallength lens subsystem varies.
 5. A variable focal position opticalsubsystem as claimed in claim 1, wherein a focal position of saidsubsystem varies in accordance with the following formula: ##EQU8##wherein S represents a distance between a rear side principal point ofthe fixed focal length lens and the focal position of said system, f_(F)is the focal length of the fixed focal length lens subsystem, and f_(V)is the focal length of the variable focal length lens subsystem.
 6. Avariable focal position optical system as claimed in claim 1, whereinthe variable focal length lens subsystem is positioned with respect tosaid fixed focal length lens subsystem such that a variation in thefocal length of the variable focal length lens subsystem changes thebeam waist position of the incident light beam being emitted by theoptical system without changing the beam waist diameter of the incidentlight beam.
 7. A variable focal position optical system comprising:afirst variable focal length lens subsystem which has lens power in afirst direction orthogonal to an optical axis and whose focal length isvariable; a second variable focal length lens subsystem which has lenspower in a second direction, which is orthogonal to the optical axis andwhich is different than said first direction, and whose focal length isvariable; a first fixed focal length lens subsystem which has lens powerin said first direction and whose focal length is fixed; and a secondfixed focal length lens subsystem which has lens power in said seconddirection and whose focal length is fixed, wherein said first variablefocal length lens subsystem and said first fixed focal length lenssubsystem are arranged such that a distance between a principal point ofsaid first variable focal length lens subsystem and a principal point ofsaid first fixed focal length lens subsystem is substantially equal tothe focal length of said first fixed focal length lens subsystem, andsaid second variable focal length lens subsystem and said second fixedfocal length lens subsystem are arranged such that a distance between aprincipal point of said second variable focal length lens subsystem anda principal point of said second fixed focal length lens subsystem issubstantially equal to the focal length of said second fixed focallength lens subsystem.
 8. A variable focal position optical systemaccording to claim 7, wherein each of said variable focal length lenssubsystems includes an electrooptic medium having an electroopticaleffect.
 9. A light beam scanning apparatus comprising:a scanning opticalsystem which scans a light beam irradiated from a light source onto anobject to be illuminated; a variable focal position optical system whosefocal position is variable; and a control device which controls thefocal position of said variable focal position optical system such thata beam waist position of the light beam scanned onto said objectsubstantially coincides with a surface of said object, wherein saidvariable focal position optical system includes a variable focal lengthlens subsystem whose focal length is variable and a fixed focal lengthlens subsystem whose focal length is fixed, said variable focal lengthlens subsystem and said fixed focal length lens subsystem being arrangedsuch that a distance between a principal point of said variable focallength lens subsystem and a principal point of said fixed focal lengthlens subsystem is substantially equal to the focal length of said fixedfocal length lens subsystem.
 10. A light beam scanning apparatuscomprising:a scanning optical system which scans a light beam irradiatedfrom a light source onto an object to be illuminated; a variable focalposition optical system whose focal position is variable; and a controldevice which controls the focal position of said variable focal positionoptical system such that a beam waist position of the light beam scannedonto said object substantially coincides with a surface of said object,wherein said variable focal position optical system includes a firstvariable focal length lens subsystem which has lens power in a firstdirection orthogonal to an optical axis and whose focal length isvariable, a second variable focal length lens subsystem which has lenspower in a second direction, which is orthogonal to the optical axis andwhich is different than said first direction, and whose focal length isvariable, a first fixed focal length lens subsystem which has lens powerin said first direction and whose focal length is fixed, and a secondfixed focal length lens subsystem which has lens power in said seconddirection and whose focal length is fixed, said first variable focallength lens subsystem and said first fixed focal length lens subsystembeing arranged such that a distance between a principal point of saidfirst variable focal length lens subsystem and a principal point of saidfirst fixed focal length lens subsystem is substantially equal to thefocal length of said first fixed focal length lens subsystem, and saidsecond variable focal length lens subsystem and said second fixed focallength lens subsystem are arranged such that a distance between aprincipal point of said second variable focal length lens subsystem anda principal point of said second fixed focal length lens subsystem issubstantially equal to the focal length of said second fixed focallength lens subsystem.